1. Field of the Invention
The present invention relates to a ladder type filter. More specifically, the present invention relates to such ladder type filter that comprises a plurality of piezoelectric resonators disposed in two columns in a casing.
2. Description of the Prior Art
FIG. 1 is a perspective view of a casing for constituting a ladder type filter, which constitutes the background of the invention. FIG. 2A is a plan view showing a plurality of piezoelectric resonators housed in the FIG. 1 casing. FIG. 2B is a longitudinal sectional view taken along the line IIB--IIB in FIG. 2A. FIG. 3 is a perspective view showing a piezoelectric resonator included in the FIG. 2A ladder type filter. FIG. 4 is a view showing one example of a terminal plate for electrically connecting a plurality of piezoelectric resonators included in the FIG. 2A ladder type filter. FIG. 5 is an equivalent circuit diagram of a ladder type filter which constitutes the background of the invention. FIG. 6 is a view showing a bandpass characteristic of a ladder type filter which constitutes the background of the invention.
Now referring to FIGS. 1 to 6, a conventional ladder type filter will be described. A casing 11 of a rectangular parallelepiped is made of synthetic resin. The upper portion of the casing 11 is formed with an opening. The casing 11 is also provided with an input terminal 131, an output terminal 132 and ground terminals 133 and 134 at four corners thereof. A plurality of piezoelectric resonators 121-125 are housed in the casing 11. As shown in FIG. 3, each piezoelectric resonator (referred to generally as resonator 12) comprises a square piezoelectric body 12a and electrode films 12b and 12c formed on the entire surface of both major surfaces. Alternatively, the electrode films 12b and 12c each may be formed only at the central portion of the piezoelectric body 12a. Piezoelectric resonators 121, 123 and 125 are relatively thick and piezoelectric resonators 122 and 124 are relatively thin. By thus differently selecting the thickness of the piezoelectric vibrators, a capacitance ratio is achieved between the respective piezoelectric resonators 12 and hence the maximum amount attenuation of the filter can be improved.
Furthermore, a plurality of terminal plates 141 to 150 are prepared for the purpose of electrically connecting piezoelectric resonators 121 to 125. As shown in FIG. 4, each terminal plate (referred to generally as plate 14) is formed in substantially the same square shape as that of the piezoelectric resonator 12. A protruding portion 14a is formed at the central portion of the terminal plate 14 so as to be in contact with the electrode film of the respective piezoelectric resonator 121 to 125 to which it is connected. A portion of the terminal plates 14 out of the plurality of terminal plates 14 have strip like connecting portions 14b formed extending outward from one side end thereof. The connecting portions 14b are used for connection to the input terminal 131 and other terminal plates 14.
The above described piezoelectric resonators 121, 123 and 125 of a relatively large thickness and the piezoelectric resonators 122 and 124 of a relatively small thickness are alternately disposed so that the respective major surfaces face each other. The piezoelectric resonators 121 to 125 are electrically connected by means of the terminal plates 141 to 150. More specifically, the electrode film of one major surface of the piezoelectric resonator 121 is in contact with the terminal plate 141 and the terminal plate 141 is connected to the input terminal 131 through the connecting portion thereof. The electrode film on the other major surface of the piezoelectric resonator 121 is connected to the electrode film on one major surface of the piezoelectric resonator 122 by means of the terminal plates 142 and 143. The electrode film on the other major surface of the piezoelectric resonator 122 is in contact with the terminal plate 144 and the terminal plate 144 is connected to a ground bar 18. An insulating spacer 151 is interposed between the terminal plates 144 and 145 for the purpose of electrically insulating the same. The terminal plate 145 is in contact with the electrode film on one major surface of the piezoelectric resonator 123 and is connected to the above described terminal plate 142. The electrode film on the other major surface of the piezoelectric resonator 123 is connected to the electrode film on one major surface of the piezoelectric resonator 124 by means of the terminal plates 146 and 147. The electrode film on the other major surface of the piezoelectric resonator 124 is in contact with the terminal plate 148 and the terminal plate 148 is connected to the ground bar 18. An insulating spacer 152 is disposed between the terminal plates 148 and 149 for the purpose of electrically insulating the same. The terminal plate 149 is in contact with the electrode film on one major surface of the piezoelectric resonator 125 and is connected to the above described terminal plate 146. The electrode film on the other major surface of the piezoelectric resonator 125 is in contact with the terminal plate 150 and the terminal plate 150 is connected to the output terminal 132.
Thus, the respective piezoelectric resonators 121 to 125 and the terminal plates 141 to 150 housed in the casing 11 are blocked by means of a blocking member 16 for blocking the resonators from slipping out of the casing 11. In other words, the blocking member 16 is selected to be of the same length as that of the casing 11 in which the piezoelectric resonators 121 to 125 are arranged and is formed with protruding portions 161 and 162 at both end portions thereof. On the other hand, a pair of opposing side portions of the casing 11 are formed with recesses 111 and 112 so that the above described protruding portions 161 and 162 of the blocking member 16 may fit thereinto. When the protruding portions 161 and 162 of the blocking member 16 are fitted in the recesses 111 and 112 of the casing 11, the piezoelectric resonators 121 to 125 and the terminal plates 141 to 150 are blocked by the blocking member 16 from slipping out of the casing 11.
An equivalent electric circuit diagram of the ladder type filter 1 thus structured is shown in FIG. 5, in which the three piezoelectric resonators 121, 123 and 125 are connected in series between the input terminal 131 and the output terminal 132, while the piezoelectric resonator 122 is connected between the junction of the piezoelectric resonators 121 and 123 and the ground terminals 133 and 134 and the piezoelectric resonator 124 is connected between the junction of the piezoelectric resonators 123 and 125 and the ground terminals 133 and 134. The bandpass characteristic of such ladder type filter 1 is shown in FIG. 6, wherein the curve is relatively high in the middle frequency region, with the intermediate frequency of 455 kHz approximately at the center, and the curve abruptly falls at the higher and lower regions.
Meanwhile, with the thus structured conventional ladder type filter 1, it was the most common approach to provide an opening on the upper portion of the casing in order to facilitate housing of the piezoelectric resonators 121 to 125 and to facilitate connection between the terminal plates 141 and the input terminal 131. However, formation of an opening on the upper portion of the casing 11 makes it necessary that the connection between the terminal plates 142 and 145, the connection between the terminal plates 146 and 149 and the connection between the terminal plates 144 and 148 and the ground bar 18 all occur on the upper portion of the casing 11, which increases the height of the casing 11. Furthermore, since it is necessary to fix the blocking member 16 on the upper portion of the casing 11 so that the piezoelectric resonators 121 to 125 are prevented from slipping out of the casing 11, provision of the blocking member 16 further increases the total height of the housing.
Furthermore, the size of the casing 11 is also determined by the geometory of the piezoelectric resonators 121 to 125 being housed therein. Therefore, miniaturization of the casing 11 is restricted by the size of the piezoelectric resonators 121 to 125. More specifically, the length of the casing 11 in the direction in which the piezoelectric resonators 121 to 125 are arranged could be reduced by decreasing the thickness of the piezoelectric resonators 121 to 125 and by decreasing the thickness of the terminal plates 141 to 150. Nevertheless, an excessive decrease of the thickness of the piezoelectric resonators 121 to 125 decreases a mechanical strength thereof, resulting in fragility and degraded reliability.
Furthermore, the height of the casing 11 was not able to be decreased, without decreasing the resonance frequency of the piezoelectric resonators 121 to 125, thereby to decrease the bulkiness thereof.